1. Field of the Invention:
This invention relates to an anti-skid control apparatus for a vehicle braking system which can prevent the locking of the wheels.
2. Description of the Prior Art:
Such an anti-skid control apparatus for a vehicle braking system is known and includes a fluid pressure control valve device arranged between a master cylinder and a wheel cylinder of a brake for the wheel, the fluid pressures control valve device receives control signals of a control unit measuring the skid condition of the wheel to control the brake fluid pressure to the wheel cylinder; a hydraulic reservoir which, when the brake fluid pressure to the wheel cylinder is decreased with control of the fluid pressure control valve device, reserves the brake fluid discharged through the fluid pressure control valve device from the wheel cylinder; a pressure fluid supply conduit connecting the master cylinder with the fluid pressure control valve device; and a fluid pump for returning the brake fluid from the hydraulic reservoir to the pressure fluid supply conduit.
When a fluid pressure control valve device is provided for each of four wheels, and the fluid pressures of each of them are independently controlled, there is no problem with control operation. Also, when a fluid pressure control valve device is provided for each of front wheels and for both of rear wheels in common there is no problem with control operation. In the latter case, the one common fluid pressure control valve device is controlled on the basis of the lower one of the speeds of the rear wheels.
However either in the above cases, three or four fluid pressure control valve devices are used. Accordingly, the whole anti-skid control apparatus is large-sized and very heavy. Since the fluid pressure control valve devices are expensive, it requires high cost.
For example, the situation is considered where the brake fluid pressures of the front wheels are controlled by the two fluid pressure control valve devices in the diagonal or X-type conduit system and the brake fluid pressures of the rear wheels are controlled in common with a front wheels. However, when the vehicle runs on the road where the right and left sides are considerably different in frictional coefficient from each other, there is the problem that the one rear wheel being diagonal to the one front wheel on the higher frictional coefficient side will lock. In that case, the steering of the vehicle becomes unstable and very dangerous.
Further, the situation is considered where proportioning valves are provided for the rear wheels. However, the brake fluid pressures of the rear wheels increases in proportion to the input fluid pressures to the proportioning valves. The fear of locking is not avoided.
Accordingly, in order to provide an anti-skid control apparatus for a vehicle braking system which can be small-sized lightweight, and can avoid the problem of locking of rear wheels, this applicant previously proposed an anti-skid control apparatus for a vehicle braking system which includes; a fluid pressure control valve device arranged between a master cylinder and a wheel cylinder of a brake for the wheel, the fluid pressure control valve device receives control signals of a control unit measuring the skid condition of the wheel to control the brake fluid pressure to the wheel cylinder; a hydraulic reservoir which, when the brake fluid pressure to the wheel cylinder is decreased with control of the fluid pressure control valve device, reserves the brake fluid discharged through the fluid pressure control valve device from the wheel cylinder; a pressure fluid supply conduit connecting the master cylinder with the fluid pressure control valve device; and a fluid pump for returning the brake fluid from the hydraulic reservoir to the pressure fluid supply conduit. The fluid pressure control valve device being provided for a pair of front wheels. A valve apparatus receiving fluid pressures of wheel cylinders of the front wheels, arranged between the pair of front wheels and a pair of rear wheels, such that when any one of the fluid pressure control valve devices starts to control the fluid pressure of the rear wheel being at the same side as the one front wheel, the fluid pressure of the wheel cylinder of which is lower is controlled in accordance with the lower one of the fluid pressures of the wheel cylinders of the front wheels by the valve apparatus.
In the above-described anti-skid control apparatus, the control signals of the control unit are formed by judging the skid conditions of the respective front wheels. On the assumption that the front and rear wheels are provided with the tires of the same kind, the braking forces are so distributed to the wheels that the front wheels tend to lock sooner than the rear wheels, when the vehicle is rapidly braked on a road which is uniform in frictional coefficient.
However, when the above assumption is not fulfilled, for example, when only the front wheels are provided with spike tires or chains for running on a snow or ice road and rear wheels are provided with the normal tires, the rear wheels tend sooner to lock than the front wheels. In the above anti-skid control apparatus, the brake fluid pressure is not controlled with the locking of the rear wheel. When the brake fluid pressure of the front wheel is controlled over the limit locking pressure of the rear wheel, the locking of the rear wheel is not released such that the steering stability cannot be maintained.
Even in the case where the front and wheels are provided with the tires of the same kind, the rear wheel may tend sooner to lock than the front wheel when the frictional coefficient of the brake lining becomes excessively low due to thermal fade phenomenon in a front wheel brake apparatus and the limit lock pressure of the front wheel becomes excessively high and this situation is particularly problematic when the vehicle is rapidly braked on a high .mu. road. When a proportioning valve is used, the fluid pressure of the rear wheel is lower than that of the front wheel. However, it increases in proportion to the fluid pressure of the front wheel and reaches the limit lock pressure. The above described locking problem occurs.
FIG. 1 illustrates the above described problem. FIG. 1 shows the changes of the wheel speeds during the time when the vehicle is braked. FIG. 1B shows the control signals of the control unit. And FIG. 1C shows the changes of the brake fluid pressures of the wheels.
When the front and rear wheels are provided with the tires of the same kind, and they run on the road that is uniform in frictional coefficient, the brake fluid pressures P and P' of the front and rear wheels change with time, as shown by the solid lines in FIG. 1C, when the brake pedal is trodden at time t0. The control unit generates a brake maintaining instruction at time t1. The fluid pressure control valve device is constituted by an inlet valve and an outlet valve. The control signals consist of signals EV and AV for the inlet and outlet valves, respectively.
Although AV is still "0", EV becomes "1" at time t1. Thus, the brake fluid pressure P of the front wheel is maintained constant. The control unit generates a brake relieving instruction at time t2. Thus, EV is still "1", and AV becomes "1" from "0". As shown in FIG. 1C, the brake fluid pressure P of the front wheel decreases as shown in FIG. 1C. AV becomes "0" at time t3, while EV is still "1". Thus, the brake fluid pressure is maintained constant.
EV becomes "0" at time t4. The brake fluid pressure rises again. EV brakes again "1" at time t5. The brake fluid pressure is maintained constant. Hereafter, the brake pressure P in a stepwise fashion increases as described above. AV becomes "1" at time t6, while EV is "1". Accordingly the brake fluid pressure P decreases.
In the above-described manner, the brake fluid pressure P of the front wheel changes with time. The brake fluid pressure P' of the rear wheel is reduced by the proportioning valve and changes with time in accordance with the brake pressure P of the front wheel. The proportioning valve causes the hyteresis phenomenon by which the brake fluid pressure P' of the rear wheel changes a little later than that P of the front wheel. However, such a time lag is neglected in FIG. 1C.
Generally, a larger amount of brake fluid is required for a constant increase of brake fluid pressure in the lower pressure range under the influence of rigidity of the wheel cylinder in the rear wheel brake apparatus. Accordingly, the change range of the brake fluid pressure P' of the rear wheel is less than that of the front wheel, as shown in FIG. 1C.
The wheel speeds V, V' of the front and rear wheels change with time, as shown by the solid lines in FIG. 1A, in accordance with the above described changes of the brake fluid pressures. The preferable anti-skid control is effected. The wheel speeds are decreased without locking of the wheels.
However, when only the front wheels are provided with chains or when the thermal fade phenomenon occcurs in the front brake apparatus, the limit lock pressure of the front wheel is increased. In such a case, the brake fluid pressure P of the front wheel changes with time, as shown by dash-lines in FIG. 1A. It changes above the level of the brake fluid pressure shown by the solid line. On the other hand, the brake fluid pressure P' of the rear wheel changes beyond the rear limit lock pressure R, as shown by the dash line. Hereafter, even when the brake fluid pressure P of the front wheel is decreased, the locking of the rear wheel cannot be relieved, partly because the range of the change of the brake fluid pressure P' is less. The front wheel is prevented from locking, as shown by the dash line in FIG. 1A. However, the rear wheel is locked. The anti-skid control is not preferably effected. The steering stability is lost. A very dangerous situation results.